The present invention relates to an air-fuel control system used for controlling the operation of an engine.
One type of the air-fuel ratio control system for supplying a mixture gas of a predetermined air-fuel ratio to the combustion chamber of an engine of an automobile or like is an electronically-controlled fuel injection system. This system comprises either one injector, or as many injectors as the engine cylinders arranged on the intake manifold or throttle body of the engine. The valve-opening time of the injectors is controlled in accordance with the operating conditions of the engine to supply a mixture gas of a predetermined air-fuel ratio to the combustion chamber of the engine. The electronically-controlled fuel injection system is generally classified into two types. In an intake air amount type, a basic injection amount is determined in accordance with the engine intake air amount and the engine. An intake mainfold pressure type, determining a basic injection amount in accordance with the engine intake manifold pressure and engine speed.
On the other hand, a method for finely controlling the air-fuel ratio of the mixture gas supplied to the engine in accordance with the operating conditions of the engine is disclosed in Japanese Patent Publication Laid-Open No. 59330/83. In the system disclosed in the publication, the air-fuel ratio controlled ranges from 14 to 22 in accordance with the operating condition determined by engine speed and intake manifold pressure, thus covering a region for control with a leaner mixture gas than that of the stoichiometric air-fuel ratio.
The fuel enrichment at the time of acceleration of the engine comprising an air-fuel control system with such an electronically-controlled fuel injection system as mentioned above, as described in Japanese Patent Publication Laid-Open No. 144632/83, is controlled by being compensated in accordance with the change rate of the engine conditions represented by the intake manifold pressure or intake air throttle valve opening. The greater the acceleration rate, the fuel enrichment is increased more to prevent dilution of the mixture gas during acceleration, so that a proper fuel enrichment is achieved as long as the air-fuel ratio is set to the stoichiometric level in steady operation.
The fuel enrichment during acceleration specified in the cited Japanese Patent Publication Laid-Open No. 144632/83, however, is conditional on the setting of a stoichiometric air-fuel ratio. If the fuel amount is increased for acceleration while the air-fuel ratio is changed between 14 and 22 in accordance with the engine operating conditions as disclosed in Japanese Patent Publication Laid-Open No. 59330/83, an acceleration from a lean mixture gas would cause a shortage of the fuel enrichment below the desired level for the low air-fuel ratio, resulting in an insufficient acceleration performance and drivability. If acceleration is started from a rich mixture gas, by contrast, the fuel enrichment would exceed the desired enrichment level, so that the mixture gas becomes excessively rich thereby to pose the problem of an abnormally increased amount of carbon monoxide in the exhaust gas.